Among elements that support a rotating body, there is a bearing. Among elements that support a linear motion body, there are a feather key, a straight-sided spline, and a ball spline.
Among them, the ball spline is an element in which balls (or needles) are placed between a spline shaft and a sleeve. The ball spline can move the sleeve along the spline shaft regardless of the length of moving distance. The balls are placed between the sleeve and the spline shaft, so that it is possible to reduce the frictional resistance.
Smooth motions are ensured by providing a small gap between the balls and the spline shaft. In the same manner, smooth motions are ensured by providing a small gap between the balls and the sleeve. This is because if there is no gap, the balls do not rotate smoothly. The gap appears as a backlash. The backlash is defined as the maximum gap occurring when the rotation is changed from normal to reverse. When the backlash is large, a shock generated when the rotation is reversed is large.
Therefore, the backlash needs to be set to an appropriate value or an appropriate range.
As a prerequisite of the setting, the backlash needs to be measured, so that various measurement techniques have been proposed (for example, see Patent Literature 1).
The technique disclosed in Patent Literature 1 will be described with reference to FIGS. 18 to 19 hereof.
FIG. 18 is a cross-sectional view of a major part of the disclosed belt-type continuously variable transmission. The belt-type continuously variable transmission 100 abbreviated as CVT includes a fixed sheave 101, a movable sheave 103 that is movably fitted into a shaft 102 of the fixed sheave 101, an actuator 104 that moves the movable sheave 103, and a belt 105 provided so as to be sandwiched by the fixed sheave 101 and the movable sheave 103.
When the movable sheave 103 is pushed out to the fixed sheave 101 by the actuator 104, a rotation diameter of the belt 105 increases. On the other hand, when the movable sheave 103 is pulled away from the fixed sheave 101, the rotation diameter of the belt 105 decreases. The speed is varied by changing the rotation diameter.
A ball spline mechanism is arranged between the shaft 102 of the fixed sheave 101 and the movable sheave 103. The ball spline mechanism includes an inner spline groove 107 provided in the shaft 102, an outer spline groove 109 provided in a sleeve 108, and a ball 111 fitted between the grooves 107 and 109. The backlash is measured for such a ball spline mechanism.
FIG. 19 is a diagram for explaining a basic principle of a conventional backlash measurement. A reference ball 113 having a diameter smaller than that of a normal ball 111 is prepared and the reference ball 113 is placed between the inner spline groove 107 and the outer spline groove 109.
Subsequently, when the sleeve 108 is moved with respect to the shaft 102 in the axis direction of the shaft 102, the moving distance is (A0/2). The outer diameter d0 of the reference ball 113 is already known. The outer diameter d of the ball 111 at that time (to be precise, the diameter of an inscribed circle of the inner spline groove 107 and the outer spline groove 109) is geometrically calculated from the moving distance (A0/2) and the outer diameter d0.
A ball 111 whose diameter is close to the obtained outer diameter is selected and the selected ball 111 is fitted into the inner spline groove 107 and the outer spline groove 109.
In other words, the technique of Patent Literature 1 is a technique for determining an optimal outer diameter of the ball 111. Basically, the moving distance is measured by using the reference ball 113 whose diameter is small in order to increase the moving distance (A0/2).
First, a large number of balls 111 whose diameters are different from each other need to be prepared, so that it is troublesome to store and sort the larger number of balls 111. A wrong combination occurs at a certain probability.
Further, the reference ball 113 is put in and the measurement is performed, and then the reference ball 113 is removed, and thereafter the selected ball 111 is put in, so that the number of processes increases and the production efficiency decreases.
Therefore, a technique is required which can quickly measure the backlash without using the reference ball.